1. Field of the Invention
The present invention relates to a method and an apparatus for measuring the density of steam within a steam pipe which is provided in a plant and through which steam is caused to flow. The present invention will hereinafter be described in relation to the density measurement of down-hole steam in a steam injection well in the petroleum industry as a typical industrial field to which the invention can be applied. It should however be understood that the present invention is not restricted to such specific field but can find application in steam density measurement in any plant equipped with a steam pipe which prohibits or hinders the measurement of steam density from the exterior.
2. Description of the prior Art
Although no apparatus has yet been developed for measuring the density of down-hole steam in steam injection wells in the petroleum industry, there have been proposed several approaches for measurement of the steam density within steam pipes leading to some injection wells from boilers in the enhanced oil recovery equipment installed on the ground for carrying out the steam injection process.
FIGS. 8 and 9 of the accompanying drawings show, by way of example, a steam density measuring apparatus known heretofore. In these figures, reference numeral 1 denotes a steam pipe, and numeral 2 denotes a collimator for obtaining a neutron beam, which collimator is constituted by a neutron source 8 accommodated within a neutron shield 10 and a neutron moderator 9. Further, reference numeral 3 denotes a thermal neutron detector, 4 denotes an electronic counter instrument, and 5 designates a flow of steam. The steam 5 flows through a flow nozzle 6 disposed within the steam pipe 1 to reach the top end of the steam injection well 7. In this steam density measuring apparatus, there is made use of a direct relation existing between the decay of thermal neutrons in the wet steam and mean density and void fraction of the wet steam.
More specifically, the wet steam flows through the steam pipe 1 under a significantly high pressure, about 2,500 psig. The collimator 2 incorporating the neutron source 8 of a radioisotope is disposed at one side of the steam pipe 1 while the thermal neutron detector is disposed in diametrical opposition to the collimator 2 relative to the steam pipe 1. The thermal neutron detector exhibits a high sensitivity to thermal and epithermal neutrons for detecting any thermal/epithermal neutrons emitted from the neutron source 8 that are transmitted or penetrate through the steam pipe 1. The output signal of the thermal neutron detector 3 is supplied to the electronic counter instrument 4 to be processed to thereby generate a signal having a magnitude that is in proportion to the count value of the thermal/epithermal neutrons indicating the density of steam flowing through the steam pipe 1. In this manner, the density of steam confined within the steam pipe can be measured.
The steam density measuring apparatus of the structure shown in FIGS. 8 and 9 is certainly effective in such applications where there is a space available for allowing the collimator 2 including the neutron moderator to be disposed in opposition to the thermal neutron detector 3 around the steam pipe 1. On the other hand, as a steam density measuring apparatus designed for use where the space for installation of the collimator and the detector mentioned above can not be secured around the steam pipe 1, there has also been proposed a structure such as the steam density measuring apparatus in which all the components thereof are located only at one side of the steam pipe 1, as shown in FIG. 10 of the accompanying drawings.
Referring to FIG. 10, the steam density measuring apparatus illustrated is so implemented as to detect those neutrons emitted from a Cf-252 type neutron source having a mean energy of 2.3 MeV that have been back scattered in the wet steam by making use of the linear relation existing between the scattering of neutrons within the wet steam and the mean density and void fraction of the wet steam. More specifically, in FIG. 10, wet steam flows within and through a steam pipe 1 under high pressure (e.g., 2,500 psig). A neutron shielding member 10 is disposed at one side of the steam pipe 1, wherein the neutral shielding member 10 encases therein a Cf-252 type neutron source 8, a cadmium plate 11 and a BF-3 type neutron detector (termed BF-3 detector hereafter) 3. The BF-3 detector 3 has an enhanced sensitivity for thermal/epithermal neutrons and is capable of detecting those thermal/epithermal neutrons which are back scattered from the steam pipe 1. The neutron shielding member 10 serves to shield the neutrons emitted from the Cf-252 type neutron source for protecting those persons who are working in the vicinity against exposure to radiation. On the other hand, the cadmium plate 11 functions to absorb those thermal neutrons which experience moderating and scattering and would otherwise penetrate through the neutron shielding member 10 (and become noise sources). The signal obtained from the BF-3 detector 3 can then be processed in the same manner as described above in conjunction with the steam density measuring apparatus shown in FIGS. 8 and 9.
As is apparent from the foregoing, the steam density measuring apparatuses known heretofore are designed to measure the density of steam within a steam pipe in a plant installed above ground and require in any case a space which allows the measuring apparatus to be disposed at least at one side of the steam pipe. Thus, it is impossible to use the prior art steam density measuring apparatus in a physically limited environment such as a steam injection well where the measurement of the density of down-hole steam is needed.